Electrophysiologically measured visual capacities survive to a surprising degree when the great majority of visual cells has disappeared in rats of the RCS type afflicted with hereditary retinal degeneration. This includes the mass potentials of the primary visual cortex in response to diffuse light stimuli and in response to black-white gratings. Impulse activity of optic nerve axons in response to spots of light provide unequivocal evidence of retinal photoreception at a stage of advanced degeneration. The number of units of fast and intermediate conduction speeds in little decreased at the age of 6-7 months and their receptive field size and antagonistic organization differs little from normal. It is proposed that surviving cone-like cells, lacking inner and outer segments, are the photosensitive elements. It is planned to obtain data on the number of functional information channels and on the receptive field organization on retina and lateral geniculate body and to establish by histochemical means that the surviving cone cells lacking outer and inner segments are the photoreceptive elements of the abnormal retina. The influence of light on the survival rate of these remnant cone cells will be studied. They decrease in number after the age of 8 months simultaneously with the deterioration of visual function as measured recently by electrophysiological and behavioral tests.